Method for controlling a polymerization rate of styrene resins

A method for controlling a polymerization rate, in a continuous radical polymerization of styrene monomer or a mixture of styrene monomer and at least one monomer copolymerizable therewith using a complete mixing type reactor, without substantially changing an average molecular weight of a polymer formed, comprising the steps of:

• (a) keeping a polymerization temperature substantially constant at t(.degree.C.),
• (b) setting a feeding rate of the styrene monomer or the mixture of styrene monomer and at least one monomer copolymerizable therewith to A times larger rate than that in a polymerization conducted in the absence of a radical polymerization initiator, and
• (c) controlling an amount of the radical polymerization initiator added in the range of from (0.1/n).times.10.sup.-3 to (2.0/n).times.10.sup.-3 mol per 1 liter of the monomer fed, n being a number of radical generating functional groups present in one molecule of the radical polymerization initiator, such that a polymer concentration x satisfies the mathematical expression:
• x.gtoreq.0.3t+6.multidot.(A-1)+12 and that x be kept substantially constant, wherein A represents a ratio of a polymerization rate after being controlled to a rate of polymerization conducted in the absence of the radical polymerization initiator at the polymerization temperature t(.degree.C.) and the polymer concentration x in a polymerization liquid.